Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Often, the illumination devices for such applications are required to provide white light. White LED lights (i.e., lights which are perceived as being white or near-white) have been investigated as potential replacements for white incandescent lamps. A representative example of a white LED lamp includes a package of a blue light emitting diode chip, made of indium gallium nitride (InGaN) or gallium nitride (GaN), coated with a phosphor such as YAG. In such an LED lamp, the blue light emitting diode chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. In some designs, white light emitting diodes are fabricated by forming a ceramic phosphor layer on the output surface of a blue light-emitting semiconductor light emitting diode. Part of the blue ray emitted from the light emitting diode chip passes through the phosphor, while part of the blue ray emitted from the light emitting diode chip is absorbed by the phosphor resulting in emission of a yellow ray. The part of the blue light emitted by the light emitting diode which is transmitted through the phosphor is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
In another type of LED lamp, a light emitting diode chip that emits an ultraviolet ray is combined with phosphor materials that produce red (R), green (G) and blue (B) light rays. In such an “RGB LED lamp”, the ultraviolet ray that has been radiated from the light emitting diode chip excites the phosphor, causing the phosphor to emit red, green and blue light rays which, when mixed, are perceived by the human eye as white light. Consequently, white light can also be obtained as a mixture of these light rays.
Generally known in the LED manufacturing art is the difficulty of tuning wavelength converting materials so as to provide a consistent, and reproducible target color point and/or color rendering index (CRI), which is a quantitative measure of the ability of a light source to accurately reproduce color in comparison with an ideal or natural light source. One of the difficulties in tuning the wavelength converting material results from the geometric shape of the globe on which the wavelength converting material is deposited. Other difficulties, alone or in combination with the globe shape include minor variations in wavelength converting material thickness, composition, presence of defects, and the like. Any of these defects can render the manufacturing process difficult as one or more of an array of LED elements can have slightly different color points. The manifestation of these difficulties in the application of wavelength converting material is especially apparent in the construction of omni-directional LED luminaries and in particular, omni-directional “white” LED luminaries.
There is an ongoing need for ways to use solid state light emitters, e.g., LEDs, to provide white light in a wider variety of applications, with greater energy efficiency, with improved color rendering index (CRI), with improved efficacy (lm/W), low cost and/or with longer duration of service.